ORIGINAL ARTICLE Low individual-level dietary plasticity in an island-invasive generalist forager Lise Ruffino • James C. Russell • Benoı ˆt Pisanu • Ste ´phane Caut • Eric Vidal Received: 30 April 2010 / Accepted: 26 January 2011 / Published online: 26 February 2011 Ó The Society of Population Ecology and Springer 2011 Abstract The ability of invasive mammals to adjust their diet in response to new or variable resources is often pro- posed to explain their invasion success on islands with differing environmental conditions, especially islands with strong spatiotemporal changes in the nature and abundance of their resources. In this study, we investigated how habitat heterogeneity and seasonal fluctuation in resource quality affect dietary breadth and plasticity in an island- invasive rodent, the black rat Rattus rattus, on a small Mediterranean island. We tested for dietary plasticity of rats at both the individual and population levels by using traditional dietary and stable isotope analyses at succes- sively increasing time scales, coupled with a long-term study of individual rats in three habitats of close proximity. Dietary and movement analyses both indicated that R. rattus is able to exploit a wide range of resources and habitats. However, dietary plasticity and habitat breadth were far narrower at the individual level. Results revealed that rats exclusively used resources found in their local habitat, and very few individuals moved among adjacent habitats in pursuit of higher-quality resources, despite those resources being abundant in their immediate environment. This counterintuitive finding suggests that intraspecific interactions must restrict rat mobility. Our results suggest that even on small islands, accessibility of patchy and high- quality resources to individuals from the entire population is not systematic. This result has important implications when quantifying invasive rodent impacts on patchily distributed species, especially when studies use indirect methods such as dietary analyses as a substitute for direct observations of predatory behavior. Keywords Alien invasive rodents  Capture–mark–recapture  Dietary shift  Invasion success  Resource use Introduction In a constantly changing world, species must continually adapt their behavior in order to succeed in their environ- ment. Intrinsic attributes that may predispose a species to be successful are generally related to niche breadth (Ehr- lich 1989; Williamson 1996; Vasquez 2005) and may include behavioral flexibility (Sol and Lefebvre 2000; Sol et al. 2002) and dietary or habitat breadth (Ehrlich 1989; Brousseau et al. 1996; Vasquez 2005; Jeschke and Trayer 2006; Blackburn et al. 2009). According to the optimal foraging theory, generalist foragers should exploit food that maximizes the net rate of energy intake by focusing on L. Ruffino (&)  E. Vidal IMEP, CNRS-UMR 6116 & IRD 193, Universite ´ Paul Ce ´zanne, Ba ˆtiment Villemin, Domaine du Petit Arbois, Avenue Philibert, B.P. 80, 13545 Aix-en-Provence Cedex 04, France e-mail: ruffino_lise@yahoo.fr J. C. Russell Department of Environmental Science, Policy, and Management, University of California-Berkeley, Berkeley, CA 94720, USA B. Pisanu De ´partement Ecologie et Gestion de la Biodiversite ´ Conservation des espe `ces, restauration et suivi des populations, UMR 7204 MNHN-CNRS-P6, Muse ´um National d’Histoire Naturelle, 61, rue Buffon, 75231 Paris Cedex 05, France S. Caut Estacio ´n Biolo ´gica de Don ˜ana, Consejo Superior de Investigationes Cientı ´ficas (CSIC), Apdo 1056, 41080 Seville, Spain 123 Popul Ecol (2011) 53:535–548 DOI 10.1007/s10144-011-0265-6